EP0656545B1 - Magnetometer with polarized light and a coupled radiofrequency field - Google Patents

Magnetometer with polarized light and a coupled radiofrequency field Download PDF

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Publication number
EP0656545B1
EP0656545B1 EP94402724A EP94402724A EP0656545B1 EP 0656545 B1 EP0656545 B1 EP 0656545B1 EP 94402724 A EP94402724 A EP 94402724A EP 94402724 A EP94402724 A EP 94402724A EP 0656545 B1 EP0656545 B1 EP 0656545B1
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Prior art keywords
frequency
field
magnetometer
cell
polarizer
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German (de)
French (fr)
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EP0656545A1 (en
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Jean-Michel Leger
Christophe Guttin
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Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/24—Arrangements or instruments for measuring magnetic variables involving magnetic resonance for measuring direction or magnitude of magnetic fields or magnetic flux
    • G01R33/26—Arrangements or instruments for measuring magnetic variables involving magnetic resonance for measuring direction or magnitude of magnetic fields or magnetic flux using optical pumping

Definitions

  • the subject of the present invention is a light polarization field magnetometer coupled radio frequencies. She finds an application in the precise measurement of weak magnetic fields (typically in the range of 20 to 70 ⁇ T which corresponds values of the Earth's magnetic field).
  • the magnetometer of the invention enters the category of so-called resonance magnetometers, of which can find a general description in the article by F. Hartman entitled “Resonance Magnetometers”, published in the journal IEEE Transactions on Magnetics, flight. MAG-8, n ° 1, March 1972, pp. 66-75.
  • the excitement of magnetic resonance is obtained by a winding arranged around the substance used.
  • the signal pickup resonance is effected either by another winding (electric variant) either by a light beam of pumping (optical variant).
  • the document FR-A-2 663 430 describes a optically pumped magnetometer which is shown schematically in Figure 1.
  • a cell 10, at least partially transparent, is filled with a gas 12, generally of helium, at a pressure of 1 to a few torrs.
  • a light source 14 delivers a light beam 15 whose wavelength is around 1.1 ⁇ m. This beam passes through a rectilinear polarizer 16. The resulting polarized beam 17 passes through the cell 10 and emerges in the form of a bundle 18.
  • a radio frequency discharge (called “weak” or “soft”) is produced in the gas 12 by a generator 30 connected to two electrodes 32, 33 arranged around the cell 10.
  • This discharge produces atoms in a metastable being ( 2 3 S 1 in the case of helium).
  • the incident light beam 17 "pumps” these atoms from the metastable state and brings them into another excited state (2 3 P).
  • the energy levels are separated into sub-levels, called ZEEMAN.
  • a resonance between such sub-levels can be established by a radiofrequency field.
  • FIG. 1 This resonance is highlighted by various means known to those skilled in the art, including a variant is shown in Figure 1.
  • This is a winding 20 arranged on either side of the cell 10 (in a so-called Helmoltz arrangement), a radiofrequency generator 22, of a photodetector 24 receiving the emergent light radiation 18, from a amplifier 25 connected to photodetector 24, of a synchronous detector 21 receiving a reference signal from generator 22 and an integrator 23.
  • the generator 22 supplies the winding 20 with current at frequency F, which creates a field oscillating magnetic part of which maintains the resonance and modulates in return the light beam 18 having crossed cell 10. This modulation is found in the electrical signal delivered by the photodetector 24 and is detected by the detector synchronous 21.
  • the signal delivered by the detector synchronous 21 has a component in phase with the reference signal, and this component serves as a signal error.
  • the integrator 23 eliminates the error static. This error signal adjusts the frequency F of the frequency generator 22 to cancel the signal error.
  • This generator must therefore be able to be controlled in tension. It can be for example an oscillator voltage controlled (V.C.O. for short for "Voltage Controlled Oscillator ").
  • Magnetometers of this type have first used helium lamps.
  • LNA lanthanum-neodymium aluminate
  • magnetometers Although satisfactory in some respects, these magnetometers have drawbacks, however. Indeed, in principle, they are strongly anisotropic, and this in both amplitude and frequency. These anisotropies are either consequences of the cycle of optical pumping and intensity detection transmitted light, either related to the phenomenon of magnetic resonance.
  • Document FR-A-2 663 430 proposes a solution to this problem. It consists in providing the means magnetometer for rotating the direction of polarization of the beam luminous 17 injected into cell 10, to give it the optimal direction corresponding to an amplitude maximum of the resonance signal.
  • the magnetometer comprises a directional magnetometer, such as a set of three "flux-gate” or RPE magnetometer (Electronic Paramagnetic Resonance), to obtain information about the direction of the ambient field at measure.
  • a circuit for processing this information calculates the optimal orientation of the polarization and therefore controls the rotation of the polarizer.
  • the magnetometer includes means for modulating at low frequency the direction of polarization and to perform a synchronous detection of the resonance signal.
  • the signal detected serves as an error signal to correct the polarization and give it the optimal direction.
  • the polarization modulation variant has the disadvantage of reducing the band bandwidth of the magnetometer, since the maximum frequency of analysis is necessarily less than that of the modulation.
  • This type of magnetometer is illustrated on the Figure 2 which includes means already shown on Figure 1 and which therefore bear the same references: this is cell 10 filled with gas 12, the laser 14 emitting a beam 15, the polarizer 16 delivering a beam with rectilinear polarization 17, the photodetector 24 receiving the emerging beam 18, of the frequency control circuit 21, of the radio frequency generator 22, frequency meter 26 and the discharge circuit 30.
  • This magnetometer further comprises a first servo circuit 40 which controls the position angle ⁇ of the polarizer.
  • the purpose of the present invention is to remedy the drawbacks which have been set out above. To this end, it provides a magnetometer without anisotropy (frequency or amplitude) and which does not does not use polarization modulation light or directional magnetometers associates. This result is obtained by enslaving direction of polarization of the light beam incident in a direction perpendicular to the field ambient magnetic Bo to be measured and simultaneously controlling the direction of the magnetic field radio frequency parallel to the direction of polarization. In other words, the management of polarization as well as that of the radiofrequency field both are facing in the same direction whatever the direction of the field magnetic to measure.
  • the invention as defined above has the advantage of considerable simplicity compared to magnetometer described in document FR-A-2 663 430.
  • any possible source of error related to either the coding of the angular rotation of the polarizer, and its transmission to the coils, as well as any other source, is eliminated. error causing phase variations on the detected signals.
  • the polarizer and the means generating the radiofrequency field are integral with one another and the rotation of these elements is simultaneous; we are therefore sure that the same angle, preferably O °, will be kept between the direction of polarization, on the one hand, and the direction of the field B 1 radio frequency on the other hand.
  • this magnetometer will be further characterized in that the synchronous detection circuit verifies that the amplitude (A 2 ) of the signal at the double frequency is not zero and in that the direction of rectilinear polarization of the beam is parallel to the direction of the radio frequency field.
  • a magnetometer according to the invention will also be characterized in that the means generating radio frequencies consist of a single coil.
  • the magnetometer according to the invention will characterized in that the means for rotating the whole polarizer-means to apply the field of radiofrequency are constituted by a motor piezoelectric.
  • a magnetometer according to a preferred embodiment of the invention is shown in Figure 3.
  • This magnetometer includes means already shown in Figures 1 and 2 and which therefore bear the same references. These are the cell 10 filled with gas 12, the laser 14 emitting a beam 15, the polarizer 16 delivering a rectilinear polarized beam 17, the photodetector 24 receiving the emerging beam 18, the frequency control circuit 21, the radio frequency generator 22, frequency meter 26 and discharge circuit 30.
  • the generator 22 controls the current induced in a coil 56 placed near the cell 12 so as to generate in the latter a radio frequency field B 1 .
  • a rotary contact is used, for example a contact by capacitive coupling or by a transformer whose primary winding is fixed and the movable secondary winding.
  • the coil and the polarizer are mounted so that the polarization P and the field B 1 are parallel.
  • the coil-polarizer assembly is more precisely illustrated in Figure 4.
  • the polarizer 16 is mounted in the center of a ring support 60, hollowed out in its central part.
  • the coil 56 is mounted on a hollow cylinder 62. To join the two together, it suffices to mount the ring at the end of the cylinder, for example with non-magnetic screws.
  • the signal detected by the photodetector 24 comprises, in addition to a DC component A O , two components A 1 and A 2 , respectively at the frequency of LARMOR and at the frequency twice that of LARMOR.
  • each component A 1 and A 2 is complex and it is convenient to represent them by their component in phase and in phase quadrature with a reference signal V RF at the LARMOR frequency generated by the generator 22.
  • V RF reference signal
  • quadrature component has a central asymmetrical part and almost linear.
  • One of the specifications of the magnetometer is that the measurement of the magnetic field must be independent of the orientation of the sensor relative to this field. It is therefore fundamental not to bring back any magnetic element or requiring DC power at proximity to the probe, since such components would generate during rotations of the probe magnetic signatures and therefore measurement errors of the terrestrial field whose importance would vary according to the arrangement of the magnetometer.
  • the piezoelectric motor shown is illustrated in Figure 6. Operation of this type of engine is based on the exploitation of movements generated on the surface of a stator which present in the form of an "elastic" body excited in vibration. A rotor 66 pressed against the stator by the bias of a spring 68 is driven by friction by the stator.
  • the entire engine 46 has was made with a hollow pin 70 to allow the transmission of the light beam through the device. Both rotor 66 and stator 64 are annulars and the mechanical support 16 and the means 56 excitation of the radiofrequency field, is secured to the motor via the axis of the latter.
  • the mechanical support 60, 62 is thus rotated by motor 46, speed and direction control of rotation of the rotor 66 by means of electrical excitation applied to a transducer piezoelectric used to create on the surface of the stator 64 the progressive wave which is at the origin of angular movements of the rotor.
  • the references 72 and 74 respectively represent a case and its cover, reference 76 a plate, the reference 78 a protective rubber.
  • Axis 70 passes through a bearing 80.
  • the assembly is between the spring 68 and a ceramic stop 82, the reference 84 representing a support ring and the reference 86 protection of the stop.
  • the magnetometer comprises a circuit 40 for controlling the direction of the polarizer 16.
  • This circuit comprises a first synchronous detection circuit 42 receiving the electrical signal delivered by the photodetector 24 and a reference voltage V RF coming from the generator 22.
  • This circuit 42 detects the amplitude A 1q of the component in phase quadrature with the reference signal V RF .
  • this amplitude A1q serves as an error signal, which, once amplified by an amplifier 44, supplies the motor 46 which rotates the polarizer 16 in a direction such that the signal error is canceled.
  • the rotation of the polarizer 16 causes, due to the mechanical connection of the polarizer-coil, the rotation of the coil 56.
  • FIG. 7 illustrates the function fulfilled by the servo-control 40.
  • the directions are identified with respect to a system of Oxyz trirectangular axes.
  • the light beam 17 is supposed to propagate along the axis Oy.
  • the rectilinear polarization P of this beam is therefore located in the xOz plane.
  • the ambient magnetic field B 0 is directed in any direction.
  • the servo means 40 then have the function of giving the polarization the direction D 1 perpendicular to Bo.
  • the radio frequency field B 1 is assumed to be oriented parallel to D1, due to the mechanical connection between the polarizer and the coil.
  • B 1 is therefore also perpendicular to B 0 . It will be observed that, whatever the direction of B 0 , there is always a direction of the xOz plane which is perpendicular to B 0 . Now, it is certain that during a complete rotation of the polarizer 16, the direction of polarization P will pass through a 90 ° position from B 0 .
  • the minimum angle that B 0 with a line from the xOz plane is equal to ( ⁇ / 2) - ⁇ n if ⁇ n designates the angle made by the normal to the xOz plane (in other words the Oy axis) with the field B 0 ; the maximum angle is equal to ( ⁇ / 2) + ⁇ n.
  • the angle made by the direction of polarization P with B 0 will therefore vary between ( ⁇ / 2) - ⁇ n and ( ⁇ / 2) + ⁇ n. It will therefore necessarily go through ⁇ / 2.
  • the signal detected by the photodetector 14 comprises, in addition to a continuous term A o , spectral components A 1 and A 2 respectively at the frequency of LARMOR and at the frequency twice that of LARMOR.
  • FIGS. 8a to 8d show the variations of certain signals as a function of the angle ⁇ made by the direction of polarization with the direction of the magnetic field to be measured B 0 .
  • Figure 8b shows the absolute value
  • . It is canceled for ⁇ 0 and 90 ° and also for 54 °.
  • Figure 8c shows the absolute value
  • FIG. 8d shows the amplitude A 1q , in magnitude and in sign, which is the servo signal intended for the circuit 42.
  • the polarization control is therefore carried out by verifying that two conditions are simultaneously satisfied, namely A 1q zero and A 2 greater than a certain threshold.
  • This double function is ensured in circuit 40, for example by a comparator 41 which compares A 2 with a threshold s and by a logic gate Et 45 which delivers a control signal to the motor 46 only if A 2 is greater than the threshold.
  • the circuit 21 can also deliver a signal reflecting the amplitude

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  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Measuring Magnetic Variables (AREA)

Description

Domaine techniqueTechnical area

La présente invention a pour objet un magnétomètre à polarisation lumineuse et à champ de radiofréquence couplés. Elle trouve une application dans la mesure précise des champs magnétiques faibles (typiquement dans la plage de 20 à 70 µT qui correspond aux valeurs du champ magnétique terrestre).The subject of the present invention is a light polarization field magnetometer coupled radio frequencies. She finds an application in the precise measurement of weak magnetic fields (typically in the range of 20 to 70 µT which corresponds values of the Earth's magnetic field).

Etat de la technique antérieureState of the art

Le magnétomètre de l'invention entre dans la catégorie des magnétomètres dits à résonance, dont on pourra trouver une description générale dans l'article de F. Hartman intitulé "Resonance Magnetometers", publié dans la revue IEEE Transactions on Magnetics, vol. MAG-8, n°1, mars 1972, pp. 66-75.The magnetometer of the invention enters the category of so-called resonance magnetometers, of which can find a general description in the article by F. Hartman entitled "Resonance Magnetometers", published in the journal IEEE Transactions on Magnetics, flight. MAG-8, n ° 1, March 1972, pp. 66-75.

Un magnétomètre à résonance est un appareil qui, plongé dans un champ magnétique B o, délivre un signal électrique de fréquence F dont la valeur est liée à Bo par la relation dite de LARMOR : F=γBo où γ est un rapport gyromagnétique d'un électron ou d'un nucléon, selon la substance utilisée. Pour l'électron, par exemple, ce rapport est égal à 28 Hz/nT.A resonance magnetometer is a device which, immersed in a magnetic field B o , delivers an electrical signal of frequency F whose value is linked to Bo by the so-called LARMOR relation: F = γBo where γ is a gyromagnetic ratio of an electron or a nucleon, depending on the substance used. For the electron, for example, this ratio is equal to 28 Hz / nT.

L'excitation de la résonance magnétique est obtenue par un enroulement disposé autour de la substance utilisée. Le prélèvement du signal de résonance s'effectue soit par un autre enroulement (variante électrique) soit par un faisceau lumineux de pompage (variante optique). The excitement of magnetic resonance is obtained by a winding arranged around the substance used. The signal pickup resonance is effected either by another winding (electric variant) either by a light beam of pumping (optical variant).

L'invention qui va être décrite relève de la seconde variante.The invention which will be described falls under the second variant.

Le document FR-A-2 663 430 décrit un magnétomètre à pompage optique qui est représenté schématiquement sur la figure 1.The document FR-A-2 663 430 describes a optically pumped magnetometer which is shown schematically in Figure 1.

Une cellule 10, au moins partiellement transparente, est remplie d'un gaz 12, en général de l'hélium, à une pression de 1 à quelques torrs. Une source lumineuse 14 délivre un faisceau lumineux 15 dont la longueur d'onde se situe autour de 1,1 µm. Ce faisceau traverse un polariseur rectiligne 16. Le faisceau polarisé 17 qui en résulte traverse la cellule 10 et émerge sous forme d'un faisceau 18.A cell 10, at least partially transparent, is filled with a gas 12, generally of helium, at a pressure of 1 to a few torrs. A light source 14 delivers a light beam 15 whose wavelength is around 1.1 µm. This beam passes through a rectilinear polarizer 16. The resulting polarized beam 17 passes through the cell 10 and emerges in the form of a bundle 18.

Par ailleurs, une décharge radiofréquence (dite "faible" ou "douce") est produite dans le gaz 12 par un générateur 30 relié à deux électrodes 32, 33 disposées autour de la cellule 10. Cette décharge produit des atomes dans un étant métastable (23S1 dans le cas de l'hélium). Le faisceau lumineux incident 17 "pompe" ces atomes à partir de l'état métastable et les amène dans un autre état excité (23P).Furthermore, a radio frequency discharge (called "weak" or "soft") is produced in the gas 12 by a generator 30 connected to two electrodes 32, 33 arranged around the cell 10. This discharge produces atoms in a metastable being ( 2 3 S 1 in the case of helium). The incident light beam 17 "pumps" these atoms from the metastable state and brings them into another excited state (2 3 P).

En présence d'un champ magnétique B o, les niveaux d'énergie se séparent en sous-niveaux, dits de ZEEMAN. Une résonance entre de tels sous-niveaux peut être établie par un champ radiofréquence.In the presence of a magnetic field B o , the energy levels are separated into sub-levels, called ZEEMAN. A resonance between such sub-levels can be established by a radiofrequency field.

Cette résonance est mise en évidence par divers moyens connus de l'homme du métier, dont une variante est représentée sur la figure 1. Il s'agit d'un enroulement 20 disposé de part et d'autre de la cellule 10 (dans une disposition dite de Helmoltz), d'un générateur radiofréquence 22, d'un photodétecteur 24 recevant le rayonnement lumineux émergent 18, d'un amplificateur 25 relié au photodétecteur 24, d'un détecteur synchrone 21 recevant un signal de référence provenant du générateur 22 et d'un intégrateur 23.This resonance is highlighted by various means known to those skilled in the art, including a variant is shown in Figure 1. This is a winding 20 arranged on either side of the cell 10 (in a so-called Helmoltz arrangement), a radiofrequency generator 22, of a photodetector 24 receiving the emergent light radiation 18, from a amplifier 25 connected to photodetector 24, of a synchronous detector 21 receiving a reference signal from generator 22 and an integrator 23.

Le générateur 22 alimente l'enroulement 20 en courant à la fréquence F, ce qui crée un champ magnétique oscillant dont une composante entretient la résonance et module en retour le faisceau lumineux 18 ayant traversé la cellule 10. Cette modulation se retrouve dans le signal électrique délivré par le photodétecteur 24 et est détectée par le détecteur synchrone 21. Le signal délivré par le détecteur synchrone 21 possède une composante en phase avec le signal de référence, et cette composante sert de signal d'erreur. L'intégrateur 23 en élimine l'erreur statique. Ce signal d'erreur ajuste la fréquence F du générateur de fréquence 22 pour annuler le signal d'erreur. Ce générateur doit donc pouvoir être commandé en tension. Il peut s'agir par exemple d'un oscillateur commandé en tension (V.C.O. en abrégé pour "Voltage Controlled Oscillator").The generator 22 supplies the winding 20 with current at frequency F, which creates a field oscillating magnetic part of which maintains the resonance and modulates in return the light beam 18 having crossed cell 10. This modulation is found in the electrical signal delivered by the photodetector 24 and is detected by the detector synchronous 21. The signal delivered by the detector synchronous 21 has a component in phase with the reference signal, and this component serves as a signal error. The integrator 23 eliminates the error static. This error signal adjusts the frequency F of the frequency generator 22 to cancel the signal error. This generator must therefore be able to be controlled in tension. It can be for example an oscillator voltage controlled (V.C.O. for short for "Voltage Controlled Oscillator ").

Un signal électrique de résonance finit donc par s'établir dans cette boucle et ce signal est à la fréquence de LARMOR. Un fréquencemètre 26 en donne la valeur F. Le champ à mesurer B o s'en déduit par la relation Bo=F/γ.An electrical resonance signal therefore ends up being established in this loop and this signal is at the LARMOR frequency. A frequency meter 26 gives the value F. The field to be measured B o is deduced from it by the relation Bo = F / γ.

Les magnétomètres de ce type ont d'abord utilisé des lampes à hélium. L'obtention récente de cristaux d'aluminate de lanthane-néodyme (ou LNA) ayant permis de réaliser des lasers accordables autour de la longueur d'onde de 1,083 µm correspondant précisément à la raie de pompage optique de l'hélium, ce type de laser s'est substitué tout naturellement à ces lampes avec une amélioration sensible des performances, ce qui a donné un regain d'intérêt à ces magnétomètres. Magnetometers of this type have first used helium lamps. The recent obtaining of lanthanum-neodymium aluminate (or LNA) crystals having allowed to realize tunable lasers around the wavelength of 1.083 µm corresponding precisely to the optical pumping line of helium, this type of laser naturally replaced these lamps with a significant performance improvement, which has given renewed interest to these magnetometers.

Un tel magnétomètre, équipé d'un laser à LNA est décrit dans le document FR-A-2 598 518.Such a magnetometer, equipped with an LNA laser is described in document FR-A-2 598 518.

Bien que satisfaisants à certains égards, ces magnétomètres présentent cependant des inconvénients. En effet, par principe, ils sont fortement anisotropes, et ceci à la fois en amplitude et en fréquence. Ces anisotropies sont soit des conséquences du cycle de pompage optique et de la détection de l'intensité lumineuse transmise, soit liées au phénomène de résonance magnétique.Although satisfactory in some respects, these magnetometers have drawbacks, however. Indeed, in principle, they are strongly anisotropic, and this in both amplitude and frequency. These anisotropies are either consequences of the cycle of optical pumping and intensity detection transmitted light, either related to the phenomenon of magnetic resonance.

Le document FR-A-2 663 430, déjà cité, propose une solution à ce problème. Elle consiste à munir le magnétomètre de moyens permettant de faire tourner la direction de polarisation rectiligne du faisceau lumineux 17 injecté dans la cellule 10, pour lui donner la direction optimale correspondant à une amplitude maximale du signal de résonance.Document FR-A-2 663 430, already cited, proposes a solution to this problem. It consists in providing the means magnetometer for rotating the direction of polarization of the beam luminous 17 injected into cell 10, to give it the optimal direction corresponding to an amplitude maximum of the resonance signal.

Plusieurs moyens peuvent être utilisés pour déterminer et obtenir cette direction optimale. Dans une première variante, le magnétomètre comprend un magnétomètre directionnel, comme par exemple un ensemble de trois "flux-gate" ou magnétomètre à RPE (Résonance Paramagnétique Electronique), pour obtenir des informations sur la direction du champ ambiant à mesurer. Un circuit de traitement de ces informations calcule l'orientation optimale de la polarisation et commande, en conséquence, la rotation du polariseur.Several means can be used to determine and obtain this optimal direction. In a first variant, the magnetometer comprises a directional magnetometer, such as a set of three "flux-gate" or RPE magnetometer (Electronic Paramagnetic Resonance), to obtain information about the direction of the ambient field at measure. A circuit for processing this information calculates the optimal orientation of the polarization and therefore controls the rotation of the polarizer.

Dans une deuxième variante, le magnétomètre comprend des moyens pour moduler à basse fréquence la direction de polarisation et pour effectuer une détection synchrone du signal de résonance. Le signal détecté sert de signal d'erreur pour corriger la polarisation et lui donner la direction optimale.In a second variant, the magnetometer includes means for modulating at low frequency the direction of polarization and to perform a synchronous detection of the resonance signal. The signal detected serves as an error signal to correct the polarization and give it the optimal direction.

Bien que donnant satisfaction à certains égards, ces solutions présentent l'inconvénient de la complexité, surtout dans la variante à flux-gates associés. La variante à modulation de polarisation présente en outre l'inconvénient de réduire la bande passante du magnétomètre, puisque la fréquence maximale d'analyse est nécessairement inférieure à celle de la modulation.Although satisfying some these solutions have the disadvantage of complexity, especially in the flux-gates variant associates. The polarization modulation variant has the disadvantage of reducing the band bandwidth of the magnetometer, since the maximum frequency of analysis is necessarily less than that of the modulation.

Une solution à ces problèmes a été proposée dans la demande EP-A-0579537 (date de publication 19.01.94) intitulée "Magnétomètre à polarisation lumineuse et à champ de radiofréquence asservis". Cette demande présente un magnétomètre sans anisotropie (de fréquence ou d'amplitude) et qui ne fait pas appel à une modulation de la polarisation lumineuse ou à des magnétomètres directionnels associés. Ce résultat est obtenu en asservissant en direction la polarisation P du faisceau lumineux incident dans une direction perpendiculaire au champ magnétique ambiant B o à mesurer et, simultanément, en asservissant la direction du champ magnétique radiofréquence B 1 parallèlement à la direction de polarisation P . En d'autres termes, la direction de polarisation P ainsi que celle du champ radiofréquence B 1 se trouvent toutes deux orientées dans la même direction optimale et ce, quelle que soit la direction du champ magnétique à mesurer.A solution to these problems has been proposed in application EP-A-0579537 (publication date 19.01.94) entitled "Magnetometer with light polarization and controlled radio frequency field". This request presents a magnetometer without anisotropy (of frequency or amplitude) and which does not use a modulation of the light polarization or associated directional magnetometers. This result is obtained by slaving in direction P polarization of the incident light beam in a direction perpendicular to the ambient magnetic field. B o to measure and simultaneously slaving the direction of the radiofrequency magnetic field B 1 parallel to the direction of polarization P . In other words, the direction of polarization P as well as that of the radiofrequency field B 1 are both oriented in the same optimal direction, regardless of the direction of the magnetic field to be measured.

Ce type de magnétomètre est illustré sur la figure 2 qui comprend des moyens déjà représentés sur la figure 1 et qui portent pour cette raison les mêmes références : il s'agit de la cellule 10 remplie de gaz 12, du laser 14 émettant un faisceau 15, du polariseur 16 délivrant un faisceau à polarisation rectiligne 17, du photodétecteur 24 recevant le faisceau émergeant 18, du circuit d'asservissement de fréquence 21, du générateur de radiofréquence 22, du fréquencemètre 26 et du circuit de décharge 30. This type of magnetometer is illustrated on the Figure 2 which includes means already shown on Figure 1 and which therefore bear the same references: this is cell 10 filled with gas 12, the laser 14 emitting a beam 15, the polarizer 16 delivering a beam with rectilinear polarization 17, the photodetector 24 receiving the emerging beam 18, of the frequency control circuit 21, of the radio frequency generator 22, frequency meter 26 and the discharge circuit 30.

Ce magnétomètre comporte en outre un premier circuit d'asservissement 40 qui pilote la position angulaire  du polariseur.This magnetometer further comprises a first servo circuit 40 which controls the position angle  of the polarizer.

Il comporte aussi un circuit d'asservissement 50 du champ de radiofréquence B 1 qui comprend :

  • un système de mesure et de codage de l'angle  de polarisation 52,
  • deux multiplieurs 541, 542 recevant, d'une part, ces signaux et, d'autre part, le signal radiofréquence délivré par le générateur 22,
  • deux enroulements 561, 562 à axes orthogonaux auxquels sont connectées les sorties des deux multiplieurs 541 et 542.
It also includes a servo circuit 50 of the radiofrequency field B 1 which includes:
  • a system for measuring and coding the angle  of polarization 52,
  • two multipliers 54 1 , 54 2 receiving, on the one hand, these signals and, on the other hand, the radiofrequency signal delivered by the generator 22,
  • two windings 56 1 , 56 2 with orthogonal axes to which the outputs of the two multipliers 54 1 and 54 2 are connected.

En injectant, dans les enroulements des courants i1=icos et i2=isin, on peut ajuster la direction de B 1 sur celle de la polarisation P .By injecting, in the windings of the currents i 1 = icos and i 2 = isin, we can adjust the direction of B 1 on that of polarization P .

Ce montage génère plusieurs types d'erreurs qui modifient ou affectent l'angle ( P , B 1) que l'on souhaiterait, au contraire, maintenir constant et égal à 0° puisque, lorsque les deux directions P et B 1 sont perpendiculaire à B o, un fonctionnement optimum du magnétomètre est assuré. Ces erreurs proviennent :

  • 1. d'erreurs au cours du codage de l'angle  (par le système (52), voir figure 2). En fait, il faudrait une précision du codeur de l'ordre du millidegré, ce qui est irréaliste parce que trop complexe à réaliser,
  • 2. d'erreur sur l'orthogonalité des bobines 561 et 562 qu'il faudrait pouvoir maintenir strictement perpendiculaires,
  • 3. d'erreurs ou de variations des fonctions de transfert courant fourni aux bobines/champ engendré par les bobines.
    • This assembly generates several types of errors which modify or affect the angle ( P , B 1 ) that we would, on the contrary, wish to maintain constant and equal to 0 ° since, when the two directions P and B 1 are perpendicular to B o , optimum functioning of the magnetometer is ensured. These errors come from:
  • 1. errors during the coding of the angle  (by the system (52), see figure 2). In fact, it would require a precision of the coder of the order of the millidegre, which is unrealistic because too complex to achieve,
  • 2. error on the orthogonality of the coils 561 and 562 which should be able to be kept strictly perpendicular,
  • 3. errors or variations in the current transfer functions supplied to the coils / field generated by the coils.

    • Or, il a été constaté que des modifications de l'angle ( P , B 1) se traduisent par des variations de la phase du signal de résonance utilisé pour mesurer la fréquence de LARMOR.However, it was found that modifications of the angle ( P , B 1 ) result in variations in the phase of the resonance signal used to measure the LARMOR frequency.

    Ainsi, d'une variation de cette phase de un degré, il résulte un décalage de fréquence du magnétomètre de l'ordre du nanotesla alors que la sensibilité à atteindre est de l'ordre du picotesla.So from a variation of this phase by one degree it results in a frequency shift of the magnetometer of the order of nanotesla while the sensitivity to reach is of the order of picotesla.

    Le magnétomètre proposé par cette demande FR-92 08783, quoique satisfaisante à certains égards, n'en comporte donc pas moins certains désavantages.The magnetometer proposed by this request FR-92 08783, although satisfactory in certain respects, there are therefore certain disadvantages.

    Exposé de l'inventionStatement of the invention

    La présente invention a justement pour but de remédier aux inconvénients qui ont été exposés ci-dessus. A cette fin, elle prévoit un magnétomètre sans anisotropie (de fréquence ou d'amplitude) et qui ne fait pas appel à une modulation de la polarisation lumineuse ou à des magnétomètres directionnels associés. Ce résultat est obtenu en asservissant en direction de la polarisation du faisceau lumineux incident dans une direction perpendiculaire au champ magnétique ambiant Bo à mesurer et, simultanément, en asservissant la direction du champ magnétique radiofréquence parallèlement à la direction de polarisation. En d'autres termes, la direction de polarisation ainsi que celle du champ radiofréquence se trouvent toutes deux orientées dans la même direction optimale et ce, quelle que soit la direction du champ magnétique à mesurer. The purpose of the present invention is to remedy the drawbacks which have been set out above. To this end, it provides a magnetometer without anisotropy (frequency or amplitude) and which does not does not use polarization modulation light or directional magnetometers associates. This result is obtained by enslaving direction of polarization of the light beam incident in a direction perpendicular to the field ambient magnetic Bo to be measured and simultaneously controlling the direction of the magnetic field radio frequency parallel to the direction of polarization. In other words, the management of polarization as well as that of the radiofrequency field both are facing in the same direction whatever the direction of the field magnetic to measure.

    De façon plus précise, la présente invention a pour objet un magnétomètre à résonance et à pompage optique comprenant une cellule remplie d'un gaz dont les atomes présentent un rapport gyromagnétique γ, cette cellule plongeant dans un champ magnétique ambiant B o qui est le champ à mesurer, une source lumineuse émettant un faisceau lumineux, un polariseur rectiligne traversé par ce faisceau et donnant à ce faisceau une polarisation rectiligne selon une première direction, le faisceau polarisé rectilignement traversant ensuite la cellule, un photodétecteur recevant le faisceau lumineux ayant traversé la cellule, ce photorécepteur délivrant un signal électrique, des moyens pour appliquer un champ radiofréquence B 1 à la cellule, ce champ ayant une seconde direction et une certaine fréquence, des moyens pour asservir cette fréquence à la fréquence de LARMOR F=γBo, un moyen pour mesurer cette fréquence, l'amplitude du champ magnétique ambiant B o se déduisant de la fréquence F par la relation Bo=F/γ,
    ce magnétomètre étant caractérisé par le fait qu'il comprend en outre :

    • des moyens de solidarisation mécanique solidarisant mécaniquement avec le polariseur les moyens pour appliquer le champ radiofréquence B 1,
    • des moyens d'asservissement agissant sur les moyens de solidarisation mécanique, ces moyens comprenant un circuit de détection synchrone recevant le signal électrique délivré par le photodétecteur et un signal de référence (VRF) à la fréquence F du champ radiofréquence, le signal électrique comprenant deux composantes A1 et A2 respectivement à la fréquence de Larmor et la fréquence double de celle de Larmor, le circuit détectant l'amplitude |A1q| de la composante A1 à la fréquence de Larmor F, en quadrature de phase avec le signal de référence (VRF), cette amplitude servant à commander des moyens pour faire tourner les moyens de solidarisation mécanique de sorte que, par conséquent, le polariseur et les moyens pour appliquer le champ radiofréquence sont tournés simultanément.
    More specifically, the present invention relates to a resonance and optical pumping magnetometer comprising a cell filled with a gas whose atoms have a gyromagnetic ratio γ, this cell immersing in an ambient magnetic field B o which is the field to be measured, a light source emitting a light beam, a rectilinear polarizer crossed by this beam and giving this beam a rectilinear polarization in a first direction, the rectilinearly polarized beam then passing through the cell, a photodetector receiving the beam light having passed through the cell, this photoreceptor delivering an electrical signal, means for applying a radiofrequency field B 1 to the cell, this field having a second direction and a certain frequency, means for controlling this frequency to the frequency of LARMOR F = γBo, a means for measuring this frequency, the amplitude of the ambient magnetic field B o deducing from the frequency F by the relation Bo = F / γ,
    this magnetometer being characterized in that it further comprises:
    • mechanical fastening means mechanically fastening with the polarizer the means for applying the radiofrequency field B 1 ,
    • servo means acting on the mechanical fastening means, these means comprising a synchronous detection circuit receiving the electrical signal delivered by the photodetector and a reference signal (V RF ) at the frequency F of the radiofrequency field, the electrical signal comprising two components A 1 and A 2 respectively at the frequency of Larmor and the frequency twice that of Larmor, the circuit detecting the amplitude | A 1q | of the component A 1 at the frequency of Larmor F, in phase quadrature with the reference signal (V RF ), this amplitude serving to control means for rotating the mechanical fastening means so that, consequently, the polarizer and the means for applying the radiofrequency field are rotated simultaneously.

    L'invention telle que définie ci-dessus possède l'avantage d'une simplicité considérable par rapport au magnétomètre décrit dans le document FR-A-2 663 430.The invention as defined above has the advantage of considerable simplicity compared to magnetometer described in document FR-A-2 663 430.

    En effet, elle ne fait appel ni à une modulation de la polarisation lumineuse, ni à un magnétomètre directionnel associé.Indeed, it does not call upon nor a modulation of light polarization, nor to a magnetometer associated directional.

    Par rapport à la solution qui a été proposée dans la demande FR-92 08783, on élimine toute source possible d'erreur liée soit au codage de la rotation angulaire du polariseur, et à sa transmission aux bobines, ainsi que toute autre source d'erreur entraínant des variations de phase sur les signaux détectés. En effet, dans la présente invention, le polariseur et les moyens générateurs du champ de radiofréquence sont solidaires l'un de l'autre et la rotation de ces éléments est simultanée ; on est donc sûr que le même angle, de préférence O°, sera conservé entre la direction de polarisation, d'une part, et la direction du champ B 1 de radiofréquence d'autre part.Compared to the solution which was proposed in application FR-92 08783, any possible source of error related to either the coding of the angular rotation of the polarizer, and its transmission to the coils, as well as any other source, is eliminated. error causing phase variations on the detected signals. In fact, in the present invention, the polarizer and the means generating the radiofrequency field are integral with one another and the rotation of these elements is simultaneous; we are therefore sure that the same angle, preferably O °, will be kept between the direction of polarization, on the one hand, and the direction of the field B 1 radio frequency on the other hand.

    Il en découle une erreur de phase qui est sans conséquence pour le magnétomètre dans la mesure où seules les variations du champ magnétique - et non sa valeur absolue - sont intéressantes.This results in a phase error which is without consequence for the magnetometer since only variations in the magnetic field - not its value absolute - are interesting.

    De préférence, ce magnétomètre sera de plus caractérisé en ce que le circuit de détection synchrone vérifie que l'amplitude (A2) du signal à la fréquence double n'est pas nulle et en ce que la direction de polarisation rectiligne du faisceau est parallèle à la direction du champ de radiofréquence. Preferably, this magnetometer will be further characterized in that the synchronous detection circuit verifies that the amplitude (A 2 ) of the signal at the double frequency is not zero and in that the direction of rectilinear polarization of the beam is parallel to the direction of the radio frequency field.

    Selon un autre mode de réalisation particulier, un magnétomètre selon l'invention sera de plus caractérisé en ce que les moyens générateurs de radiofréquence sont constitués d'une bobine unique.According to another particular embodiment, a magnetometer according to the invention will also be characterized in that the means generating radio frequencies consist of a single coil.

    L'utilisation d'une bobine unique apporte encore une simplification par rapport au document FR-A-2 663 430 puisqu'un mode de réalisation d'un magnétomètre y avait été proposé, où trois enroulements d'excitation étaient disposés autour de la cellule, ces enroulements étant mis en service séquentiellement par rapport à un multiplexeur.The use of a single coil brings again a simplification compared to the document FR-A-2 663 430 since an embodiment of a magnetometer had been proposed there, where three windings of excitement were arranged around the cell, these windings being put into service sequentially by compared to a multiplexer.

    Enfin, selon un autre mode de réalisation particulier, le magnétomètre selon l'invention sera caractérisé en ce que les moyens pour faire tourner l'ensemble polariseur-moyens pour appliquer le champ de radiofréquence sont constitués par un moteur piézoélectrique.Finally, according to another embodiment particular, the magnetometer according to the invention will characterized in that the means for rotating the whole polarizer-means to apply the field of radiofrequency are constituted by a motor piezoelectric.

    Le choix d'un moteur piézoélectrique a été guidé par le fait qu'il fallait trouver un moteur à la fois suffisamment puissant pour faire tourner l'ensemble polariseur-bobines de radiofréquence tout en respectant des contraintes très restrictives d'amagnétisme.The choice of a piezoelectric motor was guided by the fact that an engine had to be found at the times powerful enough to rotate the radiofrequency polarizer-coil assembly while respecting very restrictive constraints nonmagnetic.

    De toute façon, les caractéristiques et avantages de l'invention apparaítront mieux à la lumière de la description qui va suivre. Cette description porte sur des exemples de réalisation, donnés à titre explicatif et non limitatif. Elle se réfère par ailleurs à des dessins annexés sur lesquels :

    • la figure 1, déjà décrite, montre un magnétomètre de l'art antérieur,
    • la figure 2, déjà décrite, montre un mode de réalisation d'un magnétomètre selon l'art antérieur,
    • la figure 3 représente un mode de réalisation d'un magnétomètre selon l'invention,
    • la figure 4 représente le montage polariseur-bobine dans un magnétomètre selon l'invention,
    • les figures 5a et 5b représentent les composantes, en phase et en quadrature avec la tension de référence, du signal à fréquence égale à deux fois la fréquence de LARMOR,
    • la figure 6 représente un moteur piézoélectrique pour un magnétomètre selon l'invention,
    • la figure 7 est un schéma montrant l'orientation relative du champ B o et de la polarisation P ,
    • les figures 8a, 8b, 8c, 8d montrent les variations de l'amplitude de divers signaux en fonction de l'angle que fait la direction de polarisation P avec le champ magnétique ambiant B o.
    In any case, the characteristics and advantages of the invention will appear better in the light of the description which follows. This description relates to exemplary embodiments, given by way of explanation and without limitation. It also refers to attached drawings in which:
    • FIG. 1, already described, shows a magnetometer of the prior art,
    • FIG. 2, already described, shows an embodiment of a magnetometer according to the prior art,
    • FIG. 3 represents an embodiment of a magnetometer according to the invention,
    • FIG. 4 represents the polarizer-coil assembly in a magnetometer according to the invention,
    • FIGS. 5a and 5b represent the components, in phase and in quadrature with the reference voltage, of the signal at a frequency equal to twice the LARMOR frequency,
    • FIG. 6 represents a piezoelectric motor for a magnetometer according to the invention,
    • Figure 7 is a diagram showing the relative orientation of the field B o and polarization P ,
    • FIGS. 8a, 8b, 8c, 8d show the variations in the amplitude of various signals as a function of the angle made by the direction of polarization P with the ambient magnetic field B o .

    Un magnétomètre selon un mode préféré de réalisation de l'invention est représenté sur la figure 3. Ce magnétomètre comporte des moyens déjà représentés sur les figures 1 et 2 et qui portent pour cette raison les mêmes références. Il s'agit de la cellule 10 remplie de gaz 12, du laser 14 émettant un faisceau 15, du polariseur 16 délivrant un faisceau polarisé rectiligne 17, du photodétecteur 24 recevant le faisceau émergeant 18, du circuit d'asservissement de fréquence 21, du générateur de radiofréquence 22, du fréquencemètre 26 et du circuit de décharge 30. Le générateur 22 pilote le courant induit dans une bobine 56 placée à proximité de la cellule 12 de façon à engendrer dans cette dernière un champ de radiofréquence B 1. La bobine 56 et le polariseur 16 sont solidarisés mécaniquement, de telle façon que toute rotation d'angle  appliquée au polariseur entraíne une rotation du même angle de la direction du champ B 1, l'intensité de ce dernier étant définie par le générateur 22. Avantageusement, pour relier les moyens 56 et 22, on utilise un contact tournant, par exemple un contact par couplage capacitif ou par un transformateur dont l'enroulement primaire est fixe et l'enroulement secondaire mobile.A magnetometer according to a preferred embodiment of the invention is shown in Figure 3. This magnetometer includes means already shown in Figures 1 and 2 and which therefore bear the same references. These are the cell 10 filled with gas 12, the laser 14 emitting a beam 15, the polarizer 16 delivering a rectilinear polarized beam 17, the photodetector 24 receiving the emerging beam 18, the frequency control circuit 21, the radio frequency generator 22, frequency meter 26 and discharge circuit 30. The generator 22 controls the current induced in a coil 56 placed near the cell 12 so as to generate in the latter a radio frequency field B 1 . The coil 56 and the polarizer 16 are mechanically secured, so that any rotation of angle  applied to the polarizer causes a rotation of the same angle in the direction of the field B 1 , the intensity of the latter being defined by the generator 22. Advantageously, to connect the means 56 and 22, a rotary contact is used, for example a contact by capacitive coupling or by a transformer whose primary winding is fixed and the movable secondary winding.

    De préférence, la bobine et le polariseur sont montés de façon à ce que la polarisation P et le champ B 1 soient parallèles.Preferably, the coil and the polarizer are mounted so that the polarization P and the field B 1 are parallel.

    Le montage bobine-polariseur est plus précisément illustré sur la figure 4. Le polariseur 16 est monté au centre d'un support en anneau 60, évidé dans sa partie centrale. La bobine 56 est montée sur un cylindre creux 62. Pour solidariser les deux, il suffit de monter l'anneau au bout du cylindre, par exemple avec des vis amagnétiques.The coil-polarizer assembly is more precisely illustrated in Figure 4. The polarizer 16 is mounted in the center of a ring support 60, hollowed out in its central part. The coil 56 is mounted on a hollow cylinder 62. To join the two together, it suffices to mount the ring at the end of the cylinder, for example with non-magnetic screws.

    Il faut noter que le signal détecté par le photodétecteur 24 comprend, outre une composante continue AO, deux composantes A1 et A2, respectivement à la fréquence de LARMOR et à la fréquence double de celle de LARMOR. De plus, chaque composante A1 et A2 est complexe et il est commode de les représenter par leur composante en phase et en quadrature de phase avec un signal de référence VRF à la fréquence de LARMOR engendré par le générateur 22. Ainsi, on a représenté sur les figures 5a et 5b les composantes en phase et en quadrature de phase de A2 (terme à la fréquence double de celle de LARMOR) en fonction de l'écart de la fréquence d'excitation ω par rapport à la fréquence de LARMOR.It should be noted that the signal detected by the photodetector 24 comprises, in addition to a DC component A O , two components A 1 and A 2 , respectively at the frequency of LARMOR and at the frequency twice that of LARMOR. In addition, each component A 1 and A 2 is complex and it is convenient to represent them by their component in phase and in phase quadrature with a reference signal V RF at the LARMOR frequency generated by the generator 22. Thus, we represented in FIGS. 5a and 5b the phase and quadrature phase components of A 2 (term at the frequency twice that of LARMOR) as a function of the deviation of the excitation frequency ω from the frequency of LARMOR.

    On remarque que la composante en quadrature possède une partie centrale antisymétrique et quasiment linéaire. Note that the quadrature component has a central asymmetrical part and almost linear.

    Aucune variation de phase due à une variation de l'angle ( P , B 1) ne peut influer sur l'amplitude de la composante de A1 en quadrature avec la tension de référence, qui est détectée par le circuit 42 : en effet, du fait de la solidarisation polariseur-bobine, un angle constant, de préférence nul, est maintenu entre P et B 1. Or, c'est la variation de cet angle qui entraínerait en l'absence de solidarisation, une variation des phases de A1 et A2.No phase variation due to a variation of the angle ( P , B 1 ) cannot influence the amplitude of the component of A 1 in quadrature with the reference voltage, which is detected by the circuit 42: in fact, due to the polarizer-coil connection, a constant angle, preferably zero , is maintained between P and B 1 . However, it is the variation of this angle which would result in the absence of joining, a variation of the phases of A 1 and A 2 .

    Le moteur 46 avec lequel on commande le polariseur 16 sera un moteur suffisamment puissant pour pouvoir effectuer une rotation de l'ensemble polariseur-bobine-moyens de solidarisation. Mais il faut, d'autre part, respecter des contraintes très restrictives d'amagnétisme. En effet, une des spécifications du magnétomètre est que la mesure du champ magnétique doit être indépendante de l'orientation du capteur par rapport à ce champ. Il est donc fondamental de ne ramener aucun élement magnétique ou nécessitant une alimentation par courant continu à proximité de la sonde, puisque de tels composants généreraient lors de rotations de la sonde des signatures magnétiques et donc des erreurs de mesure du champ terrestre dont l'importance varierait selon la disposition du magnétomètre.The motor 46 with which the polarizer 16 will be an engine powerful enough to ability to rotate assembly polarizer-coil-securing means. But he must, on the other hand, respect very constraints restrictive of non-magnetism. One of the specifications of the magnetometer is that the measurement of the magnetic field must be independent of the orientation of the sensor relative to this field. It is therefore fundamental not to bring back any magnetic element or requiring DC power at proximity to the probe, since such components would generate during rotations of the probe magnetic signatures and therefore measurement errors of the terrestrial field whose importance would vary according to the arrangement of the magnetometer.

    Compte tenu de ces contraintes, un moteur de type piézoélectrique a été choisi.Given these constraints, an piezoelectric type was chosen.

    Plus précisément, le moteur piézoélectrique réalisé est illustré sur la figure 6. Le fonctionnement de ce type de moteur repose sur l'exploitation des mouvements générés à la surface d'un stator qui se présente sous la forme d'un corps "élastique" excité en vibrations. Un rotor 66 plaqué contre le stator par le biais d'un ressort 68 est entraíné par frottements par le stator. Pour l'invention, l'ensemble du moteur 46 a été réalisé avec un axe creux 70 afin de permettre la transmission du faisceau lumineux à travers le dispositif. Le rotor 66 et le stator 64 sont tous deux annulaires et le support mécanique 16 et les moyens 56 d'excitation du champ de radiofréquence, est solidarisé au moteur par l'intermédiaire de l'axe de ce dernier. Le support mécanique 60, 62 est ainsi mis en rotation par le moteur 46, le contrôle de la vitesse et du sens de rotation du rotor 66 se faisant par le biais de l'excitation électrique appliquée à un transducteur piézoélectrique utilisé pour créer à la surface du stator 64 l'onde progressive qui est à l'origine des déplacements angulaires du rotor. Sur la figure 6, les références 72 et 74 représentent respectivement un boítier et son couvercle, la référence 76 une platine, la référence 78 un caoutchouc de protection. L'axe 70 passe dans un palier 80. L'ensemble est compris entre le ressort 68 et une butée en céramique 82, la référence 84 représentant une bague d'appui et la référence 86 une protection de la butée.More specifically, the piezoelectric motor shown is illustrated in Figure 6. Operation of this type of engine is based on the exploitation of movements generated on the surface of a stator which present in the form of an "elastic" body excited in vibration. A rotor 66 pressed against the stator by the bias of a spring 68 is driven by friction by the stator. For the invention, the entire engine 46 has was made with a hollow pin 70 to allow the transmission of the light beam through the device. Both rotor 66 and stator 64 are annulars and the mechanical support 16 and the means 56 excitation of the radiofrequency field, is secured to the motor via the axis of the latter. The mechanical support 60, 62 is thus rotated by motor 46, speed and direction control of rotation of the rotor 66 by means of electrical excitation applied to a transducer piezoelectric used to create on the surface of the stator 64 the progressive wave which is at the origin of angular movements of the rotor. In Figure 6, the references 72 and 74 respectively represent a case and its cover, reference 76 a plate, the reference 78 a protective rubber. Axis 70 passes through a bearing 80. The assembly is between the spring 68 and a ceramic stop 82, the reference 84 representing a support ring and the reference 86 protection of the stop.

    Le magnétomètre comprend un circuit 40 d'asservissement de la direction du polariseur 16. Ce circuit comprend un premier circuit de détection synchrone 42 recevant le signal électrique délivré par le photodétecteur 24 et une tension de référence VRF provenant du générateur 22. Ce circuit 42 détecte l'amplitude A1q de la composante en quadrature de phase avec le signal de référence VRF. Pour des raisons qui apparaítront mieux en liaison avec la figure 8d, cette amplitude A1q sert de signal d'erreur, lequel, une fois amplifié par un amplificateur 44, alimente le moteur 46 qui fait tourner le polariseur 16 dans un sens tel que le signal d'erreur s'annule. La rotation du polariseur 16 entraíne, du fait de la solidarisation mécanique polariseur-bobine, la rotation de la bobine 56.The magnetometer comprises a circuit 40 for controlling the direction of the polarizer 16. This circuit comprises a first synchronous detection circuit 42 receiving the electrical signal delivered by the photodetector 24 and a reference voltage V RF coming from the generator 22. This circuit 42 detects the amplitude A 1q of the component in phase quadrature with the reference signal V RF . For reasons which will appear better in connection with FIG. 8d, this amplitude A1q serves as an error signal, which, once amplified by an amplifier 44, supplies the motor 46 which rotates the polarizer 16 in a direction such that the signal error is canceled. The rotation of the polarizer 16 causes, due to the mechanical connection of the polarizer-coil, the rotation of the coil 56.

    La figure 7 permet d'illustrer la fonction remplie par l'asservissement 40. Les directions sont repérées par rapport à un système d'axes trirectangles Oxyz. Le faisceau lumineux 17 est censé se propager le long de l'axe Oy. La polarisation rectiligne P de ce faisceau est donc situé dans le plan xOz. Le champ magnétique ambiant B 0 est dirigé dans une direction quelconque. Les moyens d'asservissement 40 ont alors pour fonction de donner à la polarisation la direction D1 perpendiculaire à Bo.FIG. 7 illustrates the function fulfilled by the servo-control 40. The directions are identified with respect to a system of Oxyz trirectangular axes. The light beam 17 is supposed to propagate along the axis Oy. The rectilinear polarization P of this beam is therefore located in the xOz plane. The ambient magnetic field B 0 is directed in any direction. The servo means 40 then have the function of giving the polarization the direction D 1 perpendicular to Bo.

    Le champ de radiofréquence B 1 est supposé orienté parallèlement à D1, du fait de la solidarisation mécanique polariseur-bobine. Lorsque la polarisation P est perpendiculaire à B 0, B 1 est donc lui aussi perpendiculaire à B 0. On observera que, quelle que soit la direction de B 0, il existe toujours une direction du plan xOz qui est perpendiculaire à B 0. Or, il est certain qu'au cours d'une rotation complète du polariseur 16, la direction de polarisation P passera par une position à 90° de B 0. En effet, l'angle minimum que fait B 0 avec une droite du plan xOz est égal à (π/2)-n si n désigne l'angle que fait la normale au plan xOz (autrement dit l'axe Oy) avec le champ B 0 ; l'angle maximum est égal à (π/2)+n. Lors d'une rotation de 360° du polariseur, l'angle que fait la direction de polarisation P avec B 0 variera donc entre (π/2)-n et (π/2)+n. Il passera donc forcément par π/2.The radio frequency field B 1 is assumed to be oriented parallel to D1, due to the mechanical connection between the polarizer and the coil. When the polarization P is perpendicular to B 0 , B 1 is therefore also perpendicular to B 0 . It will be observed that, whatever the direction of B 0 , there is always a direction of the xOz plane which is perpendicular to B 0 . Now, it is certain that during a complete rotation of the polarizer 16, the direction of polarization P will pass through a 90 ° position from B 0 . Indeed, the minimum angle that B 0 with a line from the xOz plane is equal to (π / 2) -n if n designates the angle made by the normal to the xOz plane (in other words the Oy axis) with the field B 0 ; the maximum angle is equal to (π / 2) + n. When the polarizer rotates 360 °, the angle made by the direction of polarization P with B 0 will therefore vary between (π / 2) -n and (π / 2) + n. It will therefore necessarily go through π / 2.

    Rappelons que le signal détecté par le photodétecteur 14 comporte, outre un terme continu Ao, des composantes spectrales A1 et A2 respectivement à la fréquence de LARMOR et à la fréquence double de celle de LARMOR.Recall that the signal detected by the photodetector 14 comprises, in addition to a continuous term A o , spectral components A 1 and A 2 respectively at the frequency of LARMOR and at the frequency twice that of LARMOR.

    Les figures 8a à 8d montrent les variations de certains signaux en fonction de l'angle  que fait la direction de polarisation avec la direction du champ magnétique à mesurer B 0.FIGS. 8a to 8d show the variations of certain signals as a function of the angle  made by the direction of polarization with the direction of the magnetic field to be measured B 0 .

    Sur la figure 8a, l'amplitude Ao est l'amplitude de la composante continue du signal. Cette composante est en (3cos2-1)2. Cette composante est nulle pour un angle d'environ 54°. Elle est maximum pour =90° et 0° (si =90° peut toujours être atteint, comme expliqué plus haut, ce n'est pas toujours le cas pour la valeur 0 qui ne peut être atteinte que si Bo est dans le plan xOz).In FIG. 8a, the amplitude A o is the amplitude of the continuous component of the signal. This component is in (3cos 2 -1) 2 . This component is zero for an angle of approximately 54 °. It is maximum for  = 90 ° and 0 ° (if  = 90 ° can always be reached, as explained above, this is not always the case for the value 0 which can only be reached if Bo is in the xOz plane).

    La figure 8b montre la valeur absolue |A1q| de l'amplitude de la composante à la fréquence de LARMOR et en quadrature de phase avec le signal de radiofréquence. Cette composante est en |sin2(3cos2-1|. Elle s'annule pour =0 et 90° et également pour 54°.Figure 8b shows the absolute value | A 1q | the amplitude of the component at the LARMOR frequency and in phase quadrature with the radiofrequency signal. This component is in | sin 2  (3cos 2 -1 |. It is canceled for  = 0 and 90 ° and also for 54 °.

    La figure 8c montre la valeur absolue |A2| de l'amplitude de la composante à la fréquence double de la fréquence de LARMOR. Cette composante est en |sin2(1-3cos2)|. Elle s'annule pour =0 et 54° et passe par un maximum pour =90°.Figure 8c shows the absolute value | A 2 | the amplitude of the component at the frequency double the frequency of LARMOR. This component is in | sin 2  (1-3cos 2 ) |. It is canceled for  = 0 and 54 ° and goes through a maximum for  = 90 °.

    Enfin, la figure 8d montre l'amplitude A1q, en grandeur et en signe, qui est le signal d'asservissement destiné au circuit 42.Finally, FIG. 8d shows the amplitude A 1q , in magnitude and in sign, which is the servo signal intended for the circuit 42.

    On voit que le signa A1q à la fréquence de LARMOR, par son annulation et son changement de signe pour =90°, constitue un signal d'erreur commode permettant d'obtenir un asservissement de la direction de polarisation à 90° du champ B 0. Mais ce signal s'annule également pour les angles 0 et 54° de sorte qu'il y a un risque de voir la direction de polarisation s'accrocher sur ces valeurs intempestives. Pour éviter ce risque, on utilisera conjointement le fait que le signal à fréquence double (figure 8c) présente un maximum pour =90°, ce qui permet de distinguer cette valeur des deux autres (0 et 54°) où | A2| est nul. Il n'est d'ailleurs pas nécessaire de s'assurer que le signal |A2| est maximum. Il suffit de vérifier qu'il n'est pas nul, c'est-à-dire, en pratique, qu'il excède un certain seuil.We see that the sign A1q at the LARMOR frequency, by its cancellation and its change of sign for  = 90 °, constitutes a convenient error signal making it possible to obtain a control of the direction of polarization at 90 ° from the field B 0 . But this signal is also canceled for the angles 0 and 54 ° so that there is a risk of seeing the direction of polarization catch on these untimely values. To avoid this risk, we will jointly use the fact that the double frequency signal (figure 8c) has a maximum for  = 90 °, which makes it possible to distinguish this value from the other two (0 and 54 °) where | A 2 | is zero. It is moreover not necessary to ensure that the signal | A 2 | is maximum. It suffices to verify that it is not zero, that is to say, in practice, that it exceeds a certain threshold.

    L'asservissement de polarisation s'effectue donc en vérifiant que deux conditions sont simultanément satisfaites, à savoir A1q nul et A2 supérieur à un certain seuil. Cette double fonction est assurée dans le circuit 40, par exemple par un comparateur 41 qui compare A2 à un seuil s et par une porte logique Et 45 qui ne délivre un signal de commande au moteur 46 que si A2 est supérieur au seuil.The polarization control is therefore carried out by verifying that two conditions are simultaneously satisfied, namely A 1q zero and A 2 greater than a certain threshold. This double function is ensured in circuit 40, for example by a comparator 41 which compares A 2 with a threshold s and by a logic gate Et 45 which delivers a control signal to the motor 46 only if A 2 is greater than the threshold.

    Cette propriété du signal à la fréquence double peut n'être exploitée qu'à la mise en route du magnétomètre pour accrocher la polarisation P à la valeur 90°. Ensuite, si le champ B 0 change de direction, la polarisation restera asservie à la bonne direction.This property of the signal at double frequency can only be exploited when the magnetometer is started to catch the polarization. P at the value 90 °. Then if the field B 0 changes direction, the polarization will remain subject to the right direction.

    Dès lors qu'il est fait usage du signal A2 à fréquence double, il est avantageux d'exploiter ce même signal pour asservir en fréquence le magnétomètre et cela par les moyens 21 qui viennent piloter le générateur 22. De préférence, on emploiera, pour cela, la composante de A2 en quadrature (celle représentée sur la figure 5b) par rapport au signal de référence : sa partie centrale, antisymétrique et linéaire, se prête à ce type de mesure. En outre, aucune variation de phase due à une variation de l'angle ( P , B 1) ne peut influer sur cette composante puisque, justement P , B 1 sont maintenus parallèles par la solidarisation mécanique polariseur-bobine. As soon as use is made of the signal A 2 at double frequency, it is advantageous to use this same signal to slave the magnetometer in frequency and this by the means 21 which come to drive the generator 22. Preferably, we will use, for this, the component of A 2 in quadrature (that shown in FIG. 5b) relative to the reference signal: its central part, asymmetrical and linear, lends itself to this type of measurement. In addition, no phase variation due to a variation of the angle ( P , B 1 ) cannot influence this component since, precisely P , B 1 are kept parallel by the mechanical connection between the polarizer and the coil.

    Le circuit 21 peut aussi délivrer un signal reflétant l'amplitude |A2| de la composante à la fréquence double, à destination du comparateur 41 qui exploitera cette information pour valider le signal d'erreur A1q, par l'intermédiaire du comparateur 41.The circuit 21 can also deliver a signal reflecting the amplitude | A 2 | of the component at double frequency, intended for comparator 41 which will use this information to validate the error signal A 1q , via comparator 41.

    Claims (4)

    1. Optical pumping, resonance magnetometer comprising a cell (10) filled with a gas (12), whereof the atoms have a gyromagnetic ratio γ, said cell being placed in an ambient magnetic field (Bo), which is the field to be measured, a light source (14) emitting a light beam (15), a linear polarizer (16) through which said beam (15) passes and giving said beam a linear polarization in a first direction (D1), the linearly polarized beam (17) then passing through the cell (10), a photodetector (24) receiving the light beam (18) which has passed through the cell (10), said photodetector (24) supplying an electric signal, means (56) for applying a radio frequency field (B1) to the cell (10), said field having a second direction (D2) and a certain frequency, means (21, 22) for coupling said frequency to the Larmor frequency F = γBo, a means (26) for measuring said frequency (F), the amplitude of the ambient magnetic field (Bo) being deduced from the frequency (F) by the relation Bo = F/γ, said magnetometer also being characterized in that it comprises means for mechanically joining to the polarizer (16) the means (56) for applying the radio frequency field (B1) to the cell and coupling means (40) acting on the mechanical joining means, said means having a synchronous detection circuit (42) receiving the electric signal supplied by the photodetector (24) and a reference signal (VRF) at the frequency F of the radio frequency field, the electric signal comprising two components A1 and A2 respectively at the Larmor frequency and double the Larmor frequency, the circuit (42) detecting the amplitude |A1q| of the components A1 at the Larmor frequency (F), in phase quadrature with the reference signal (VRF), said amplitude being used for controlling means (46) for rotating the mechanical joining means in such a way that as a consequence the polarizer (16) and means (56) for applying the radio frequency field are simultaneously rotated.
    2. Magnetometer according to claim 1, characterized in that the amplitude |A2| of the signal at twice the Larmor frequency is not zero and in that the rectilinear polarization direction (D1) of the beam (15) is parallel to the direction (D2) of the radio frequency field (B 1).
    3. Magnetometer according to one of the claims 1 or 2, characterized in that the radio frequency generating means (56) are constituted by a single coil.
    4. Magnetometer according to any one of the claims 1 to 3, characterized in that the means (46) for rotating the assembly constituted by the polarizer and the radio frequency field generating means (56) are constituted by a piezoelectric motor.
    EP94402724A 1993-12-01 1994-11-29 Magnetometer with polarized light and a coupled radiofrequency field Expired - Lifetime EP0656545B1 (en)

    Applications Claiming Priority (2)

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    FR9314378A FR2713347B1 (en) 1993-12-01 1993-12-01 Magnetometer with coupled light polarization and radiofrequency field.
    FR9314378 1993-12-01

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    EP0656545B1 true EP0656545B1 (en) 2002-01-30

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    CA (1) CA2136145A1 (en)
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    EP3115799A1 (en) 2015-07-08 2017-01-11 Commissariat à l'Energie Atomique et aux Energies Alternatives Isotropic and all-optical magnetometer

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    CA2136145A1 (en) 1995-06-02
    US5534776A (en) 1996-07-09
    FR2713347A1 (en) 1995-06-09
    FR2713347B1 (en) 1995-12-29
    EP0656545A1 (en) 1995-06-07
    DE69429763T2 (en) 2002-08-22
    DE69429763D1 (en) 2002-03-14

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